In general, a wide section conduit is formed by an extended tubular enclosure which may be made of molded material such as concrete or of metal. If the conduit has to resist high internal pressure, it is advantageous for its construction to be in metal as it may then be made of relatively light, easy to handle juxtaposed panels which may be welded together to form a closed tubular casing with, generally, a circular section, and which resists particularly well tensile stress generated by the application of internal pressure.
Forced conduits used in hydroelectric installations are generally made in this way.
However, the conduit must, normally, be laid on the ground, generally using spaced supports and, while it has good resistance properties when subjected to internal pressure, it also has a propensity to distort when there is no pressure, during assembly or even simply in the event of reduced pressure. Such distortion leading to ovalization of the conduit is an impediment, particularly during assembly of the different panels which, in order to avoid such distortion, must normally be mounted on a template of the desired section before being soldered.
The risk of distortion is even higher with very wide sections, for example of several square meters. Tubular components in reinforced concrete can be used which offer the advantage of keeping their shape when laid on the ground and banked, but for wide sections such components are heavy and cumbersome. Also, circular section conduits which simply rest on their lower surface exert considerable localized stress on the ground which may, through a boring effect, lead to differential subsidence and therefore disturbances. Also, it is difficult to form joints between adjacent components that are sufficiently leakproof to withstand very high pressure.
To solve such problems, the inventor has already described, in -FR-A-2,685,304 the construction of mixed conduits made of a metal tubular enclosure having a cross-section that is circular shaped on one part with a substantially level lower part which is applied and fixed on a slab of reinforced or prestressed concrete. In this manner, such a conduit combines the advantages of metal construction and concrete construction since the metal circular part is only subjected to tensile stress and may therefore be made of a relatively thin wall, whereas the lower part is applied to the slab which can be designed to withstand flexion stresses thus generated and which, also, distributes load over a wide surface area which reduces the stresses applied to the ground and risks of subsidence.
-FR-A-2,685,304 describes several particularly advantageous methods of construction.
This type of conduit fully withstands internal stresses exerted, in service, by the fluid transported even under very high pressure. However, it may be also be subjected to external stresses exerted, for example, by the filling under which the conduit is buried or simply by atmospheric pressure when internal pressure is reduced or cut off.
For better resistance against external stresses, -FR-A-2,685,304 proposes giving the conduit a half-round shape. However, the diameter of such a conduit is evidently double the diameter of a circular section conduit for the same flow rate. The conduit passage and consequently the width of the trench in which it is laid are therefore increased.
As indicated in patent FR-A-2,685,304 it is possible in certain cases to approximate a circular section as far as possible to cover a sector of more than 180.degree. whose area, for example, may reach three quadrants, the width of the concrete slab thereby being reduced.
However, by increasing the sector covered by the circular part, the risk of distortion is also increased, especially of the side panels during assembly, so that it is difficult to align these panels for welding, the use of a template being necessary in practice.
Also, this conduit must be placed on dug foundations whose levelling is not very precise. On-site handling and banking operations must be simplified as much as possible.
Also, even when the conduit is in service, mere reduction in internal pressure or even a mere hollowing effect may cause collapse of the metal part under the embankment load.